Addition agent for and method of treating steel



United States atent G ADDITION AGENT FOR AND METHOD OF TREATING STEELApplication September 15, 1955 5 Serial No. 534,620

3 Claims. (Cl. 75-57) No Drawing.

This invention relates to an addition agent for the treatment of steeland to a method of treating molten steel.

In the production of steel sheets for the manufacture of finishedproducts by pressing, stamping, drawing, etc., it has been customary,wherever possible, to use rimming steel of low carbon and preferablymoderately low manganese contend. Such steel possesses low elasticproperties, high ductility, good surface, and other desirable qualities.

When, however, deep drawing operations are contemplated, the ordinaryrimming steel of commerce is frequently inadequate. Sometimessatisfactory results are achieved by selection; by this I mean eitherselecting individual heats that are of better quality for the purposethan are other heats made in the same plant, or sometimes taking thebottom portion of each ingot and using this portion for sheets for themore difficult jobs.

An improvement over these practices has resulted from the incorporationof small amounts of vanadium of the general order of about 0.03% to0.06% in rimming steels which have had previously a partial control oftheir rimming action or evolution of gases by the addition of limitedquantities of aluminum or silico-manganese. The steels of this type inorder to retain their good surface must, of course, show on analysisonly a very small amount of silicon residual, ordinarily not more, orcertainly-not much more, than about 0.02%, so that initial deoxidationby silico-manganese must be done with care. Similarly, aluminum must notbe added to such an extent as to interfere with the proper degree of gasliberation or correct rimming. Vanadium rimming steel possesses the sameattributes as does ordinary plain carbon rimming steel in respect toductility and good surface and, additionally, appears to be capable ofundergoing considerable deep drawing, regardless of the location in theingot of the metal ultimately converted into a sheet. Also, when theconditions of the prior deoxidation are just moderately well controlledand the amount of vanadium is correct, differences between successiveheats are very considerably smaller than in the case of plain carbonsteel. From actual practice, in the-making of large tonnages, it hasbeen shown at these conditions are readily obtained.

There are, however, some jobs of a deep drawing nature that appear to betoo severe to be successfully repetitive on a production basis by theuse of the vanadium rimming steel. For these applications,steelcompletely killed by aluminum has been employed just as it wasemployed prior to the development of the vanadium rimming steel. Thealuminum killed steel is, however, subject to important defects. Theamount of metal recovered, as in the case of any killed steel, is muchsmaller than for rimmed steel or, in other words, the loss as scrap forremelting is considerably greater. Additionally, reduction of oxides inthe steel by the aluminum yields hard aluminarich inclusions that may beeither nearly pure alumina or aluminum silicates or other compounds.These appear frequently in great number and in clusters at and near theice surface and result in rejections in the steel plant on the basis ofsurface irregularities or during forming by yielding badly distorted ortorn deep drawn articles.

I have discovered that the shortcomings of the aluminum killed steelsemployed in very severe deep drawing applications can be avoided oreliminated by employing calcium and aluminum together in the deoxidationor killing of the steel. Properly proportioned, the calcium and alu-vminum yield a deoxidation product that is a calcium aluminate ofmoderately low melting point but with extremely high fluidity at steelmaking temperatures so that it rises readily into that portion of theingot that is discarded. In other words, alumina-rich inclusions, eitherindividually or in clusters, and either at the surface of a finishedsheet or deep-rooted, are no longer found in the finished product.

Minor contamination by silica due to the presence of small amounts ofthis substance resulting from oxidation of silicon in the additionagent, or by silica and other associated substances introduced byrefractories with which the molten steel comes in contact do not detractfrom the desirable qualities of the calcium aluminate oxidation productand are carried away with it out of the mass of the cooling andsolidifying steel. Expressing this fact somewhat diflerently, smallamounts of silica coming from the addition agent or the refractories, ortitania from either or both of these sources, or alkali oxides from theaddition agent or the refractories, or a small amount of unreducedmanganese oxide from the steel bath or from the alloy, do not alter thedesirable performance of the addition agent which yields an oxidationproduct preponderately of combined oxides of calcium and aluminum.

Considering only the calcium and aluminum contained in the additionagent, they should be present in the following proportions: 51 to 64%Ca, 36 to 49% Al, and may be introduced in any convenient form. Certainmetallic impurities may be present in the addition agent up to definite,critical and permissible limits. Additionally, because of itscontribution to improvements in manufacture and also to the functioningin use, there is present in the addition agent silicon to the extent ofabout 0.3% to 3.0% inasmuch as this limited quantity appears to have adesirable effect in insuring that the total of the oxides of calcium andaluminum combine to form the desired'end product. No harm appears toresult from the presence of small amounts of titanium oxide or theoxides of manganese or the alkali oxides so that in the addition agent,titanium may be present up to about 1% maximum, manganese up to about 6%maximum and the alkali metals up to about 0.5% maximum. It has beendetermined that these maxima are quite critical; above the stated level,alkali metals will tend seriously to attack ladle liningsand titanium,through its oxide, will tend to raise the melting point of the resultantslags quite substantially. Above the level given for manganese, thismetal would be added to the steel to an extent too great to satisfy somesteel specifications. Iron mayor may not be present but, ifpres'ent, thetotal of iron and manganese should not exceed" 20%. Thus, the additionagent comprises a limited amount of silicon, a low maximum of titanium,a low maximum of manganese, a low maximum of the alkali metal and up toa total amount of iron and manganese of 20%, the remainder consisting ofcalcium and aluminum in the proportions indicated above, namely, in therange of ratios of calcium to aluminum of 1.00:1 to 1.75:1.

Within the critical limits herein described, an alloy contributing thedesired properties has the following composition:

The metals' cannot satisfactorily be added individually in view of thelow specific gravity of calcium and the resultant difiiculty ofintroducing all of it beneath the surface of the molten steeleffectively and without great loss. It can be done, of course, by meansof a projecting device suchas a gun or by a plunger to force or hold thereagent in the liquid steel, but with larger masses of steel thesemeasures are inconvenient.

Accordingly, the metals can be added in the form of an alloy whichshould have the above stated proportions. However, since this alloy alsois highly reactive, an improved method comprises having an alloysomewhat richer incalcium and lower in aluminum (which is more readilyprepared) enclosed in an aluminum capsule which would thensupply some ofthe aluminum required for achieving the desired rangeof oxidecomposition; the total addition would then have the two metals in theabove preferred proportion. Iron may be. added, either in the capsule oras the: cover for an open end capsule, solely in order to add weight.Or, if preferred, an iron capsule may be used, enclosing the metals inthe desired proportion, either in the form of a crushed alloy or as theindividual metals or as a combination thereof, When the individualmetals are enclosed in such a device, they should be so comminuted thatthe reaction products can readily combine under ordinary practice ofsteel plant use.

Another suitable means of introducing these deoxidizing elements intosteel baths is that of containing the metals in a tube which may beclosed at one or both ends and which can be immersed in the steel bathto a predetermined depth, correspondingto the'volume of oxygen containedin themetal bath.

Thenew means which I have developed for deoxidizing steels to producecombined oxides having a low melting point which will separateeffectively from the metal during its cooling and not be retained in theingot after solidification, avoids the-addition of any substantialamount of silicon to the steel during deoxidation, so that at most avery minor residual silicon can result. The afiinity of calcium andaluminum for oxygen is greater than that of silicon, which is oxidizedonly after the major portion of the other two metals have been bound tooxygen. Silicon has a strong tendency to alloy with the steel. However,with the percentage of silicon in the addition agent being limited asabove described, even if none were oxidized, the amount introduced willbe well within permissible limits. Since silicon may be introduced intothe steel from other sources, if the addition agent contains more than3% silicon, there is danger that the silicon content of the steel may beraised above the level that would permit producing suitable deep-drawingsteel. At the maximum silicon content of 3%, if perchance conditions aresuch that all of the silicon is oxidized, the silica will not have adetrimental effect on the results obtained in the use of the additionagent, in that the product of the fully oxidized metals will not have ahigher melting point. The presence of the specified amount of silicon isdesired, as it appears that small amounts of silica have the eflfect ofpriming the fusion of the slag.

Furthermore, it should be noted that calcium has a very great affinityfor sulphur, and aluminum has a great aflinity for nitrogen. While thedeoxidizing alloy of my invention will ordinarily be added to steel onlyafter substantial desulphurization has taken place (if any is required),any small residual amountsof sulphur that they still remain will be atleast in part bound to the calcium.

The nitrogen content of the steel depends to a large extent onmanufacturing methods employed in its production. However, nitrogenrarely exceeds 0.01% and aluminum will combine with this nitrogen, thusbeing productive of a steel of customary ductility.

While it can, therefore, be expected that small'amounts of calcium andaluminum will be removed from the alloy to bind sulphur and nitrogen,the extent to which. this will occur is not large and will not afiectthe results'hereinlie fore described.

The oxidation products of the addition agent of this invention arestrongly basic in character, the oxidation products of an addition agentcontaining 61% calcium and 39% aluminum having a basicity factor of 0.45(that is, /a of the percentage of alumina divided by /3 of thepercentage of alumina plus the percentage of lime). By contrast, theoxidation products of the customary'deoxidizers (such as, aluminum,ferro-siliconor calcium silicide) are strongly acid in character,namely, witha: factor greater than 1.00. Thus, this featuredistinguishes the new addition agent from other deoxidation agents.

Essentially, the addition agent of my invention permits simultaneous ornearly simultaneous deoxidation ofsteel by two metals, calcium and.aluminum, and is particularly applicable to steels requiring very lowsilicon contentsi The oxides of these metals readily combine to form lowmelting calcium aluminates which have astrong tendency. to aggregateand, by virtue of these two factors, tend: to rise out of the melt andgo into: the slag- If the agent or alloy should. be" used with siliconcontainingsteels and if. some of the soft calcium aluminatesshould-accidentally be retained in the metal, they will do no damage-totools" used in machining. Thus, the invention may also-be ap:- plied inthe manufacture of killed steels for bars-,xfotgings, casings, etc.

The invention is. not limited to the preferred-embodh ment but may beotherwise embodied 'orpracticedwithin' the scope of the. followingclaims.

I. claim:

1. An addition agent for the treatment of steel, contain-; ing. 0.3 to3.0% silicon, up to 1% titanium, .up to 6% manganese, up to 0.5% alkalimetals and upto a totah amount of iron and manganese of 20% theremainder con sisting, of calcium and aluminum in therange. of ratios ofcalcium tov aluminum of 1.00:1 to 1.75:1.

2. The method of treating.v molten steel,.whiclrcorn prises adding,thereto in anamount sufiicient to. steel, an addition agent in which theprincipal.- compos nents are calcium and aluminum, the calciumbeing..between 51 and 64% and the aluminum being, between36 and.49% ofthe total of the calcium and aluminum-..

3. The method. of treating molten steel, whichco'm prises adding theretoin an amount suflicient-to kill: the steel, an addition agentaccordingto claim 1.

References Cited in the file of this patent UNITED STATES- PATENTS875,668 Me-slans Dec. 31, 1907" 1,348,458 Strasser Aug. 3, 19201,471,401 Koppers Oct. 23, 1923? 2,154,613 Guthrie Apr. 18-, 1939

1. AN ADDITION AGENT FOR THE TREATMENT OF STEEL, CONTAINING 0.3 TO 3.0%SILICON, UP TO 1% TITANIUM, UP TO 6% MANGANESE, UP TO 0.5% ALKALI METALSAND UP TO A TOTAL AMOUNT OF IRON AND MANGANESE OF 20%, THE REMAINDERCONSISTING OF CALCIUM AND ALUMINUM IN THE RANGE OF RATIOS OF CALCIUM TOALUMINUM OF 1.00:1 TO 1.75:1.